The "detect attempt to create collisions" variant of SHA-1
implementation by Marc Stevens (CWI) and Dan Shumow (Microsoft)
has been integrated and made the default.
* jk/sha1dc:
Makefile: make DC_SHA1 the default
t0013: add a basic sha1 collision detection test
Makefile: add DC_SHA1 knob
sha1dc: disable safe_hash feature
sha1dc: adjust header includes for git
sha1dc: add collision-detecting sha1 implementation